The present invention relates to a display apparatus of rear projection type (hereinafter, a rear projection display), for displaying a picture with using an image or video modulation element having pixels in the structure thereof, such as of a translucency/reflection type liquid crystal panel and/or a DMD (Digital Micro-mirror Device), as well as, a translucent type screen for use therein. In particular, it relates to a projected light therein, and to the rear-projection display apparatus and the translucent screen, in which improvements are made for reducing speckle disturbances or obstructions caused by the interferences between a diffusion element or the like, such as, diffusion materials which are contained in the translucent screen thereof.
In the conventional rear-projection display apparatus, original pictures displayed on CRTs of red, green and blue colors, each having an effective screen size of about from 5 inches to 7 inches, are enlarged to be projected upon the translucent screen, with using a projection lens of a large aperture (i.e., having a large visual angle from the screen side), the F-number of which is about 1, for example. Thus, the F-number is a numeral value indicative of brightness of the lens, and it is equal to the ratio, f/D between the focal distance xe2x80x9cfxe2x80x9d and the aperture xe2x80x9cDxe2x80x9d of the lens. The smaller the F-number, the larger the brightness of that lens. Further, the translucent screen comprises a Fresnel sheet and a lenticular sheet. On the Fresnel sheet, the Fresnel lens is formed at least upon the light exit surface thereof. On the lenticular sheet, the lenticular lenses are formed upon both surfaces thereof (i.e., the light incident surface and the light exit surface), while dispersing light diffusion material within an inside of that sheet at a certain amount thereof.
In recent years, a new product of such the rear-projection display apparatus comes up on a market, with using an image modulation element therein, which includes a reflective/translucent liquid crystal panel and plural numbers of fine mirrors, for example, in place of the CRTs. The image modulation element modulates the light emitted from a light source responding to an input video signal by a unit of the pixel, thereby to form the light of original picture. In this rear-projection display apparatus having the image modulation element, there is used or adopted a projection lens of relatively small pupil diameter (or having a small visual angle from the screen side), the F-number of which is about 2. Thus, the F-number is a numeral value indicative of brightness of the lens, and is equal to the ratio, f/D between the focal distance xe2x80x9cfxe2x80x9d and the aperture xe2x80x9cDxe2x80x9d of the lens. And, the smaller the F-number, the larger the brightness of that lens.
In a case where the visual angle of the projection lens is small, random diffuser elements, such as the diffusion materials contained within the translucent screen, interfere with the light, thereby causing the so-called speckle disturbances. The speckle disturbances is a phenomenon which can be seen often in a case where a laser light of narrow spectrum is irradiated upon the diffusion surface (for example, a frosted or obscured glass), wherein numbers of twinkling bright points are generated within a certain range in the picture irradiated, as a typical example thereof.
First, explanation will be given on a principle of generating the speckle disturbances, by referring to FIG. 1. In this FIG. 1, the projection lens 2 is that having a plural number of lens elements, for example, and it is replaced by one of the lens element for the purpose of simplification in the explanation thereof in that figure. The light exiting from the projection lens 2 is incident upon a diffusion layer 10 laminated on the translucent screen by a visual angle xcex2 (i.e., the visual angle from the screen side onto the pupil of the lens). In this instance, if the visual angle xcex2 is small, between the light ray and the random diffuser element, such as the diffusing materials, etc., which are contained within the diffusion layer 10, the mutual interferences (i.e., the speckle disturbances) occur within a certain interference extent or range D, as is shown by a waveform 12. Further, with the axes shown on the right-hand side of the waveform 12, the xe2x80x9cyxe2x80x9d axis indicates the extent or range, while the xe2x80x9czxe2x80x9d axis the strength or intensity thereof. Therefore, the extent or range is widen where the speckle disturbances occur if the xe2x80x9cyxe2x80x9d axis of the pulse-like portion is large on the waveform 12, while the intensity thereof is enhanced or increased when the xe2x80x9czxe2x80x9d axis portion is large.
In the article of SPIE society, February of 1997, regarding the range of the interferences (hereinafter, interference range) mentioned above, the followings are described. (1) The visual angle xcex2[rad] is an angle of xcex2=1/[Fxc3x97M], if assuming that the F-number of the projection lens 2 is xe2x80x9cFxe2x80x9d, and that the projection magnification of the projection lens 2 is xe2x80x9cMxe2x80x9d, and then the interference range xe2x80x9cDxe2x80x9d is in the relationship of inverse proportion to xcex2, i.e., D xcex11/xcex2. (2) The interference rangexe2x80x9cDxe2x80x9d can be established by the relationship of the following equation between the F-number xe2x80x9cFxe2x80x9d and the magnification xe2x80x9cMxe2x80x9d of the projection lens, assuming that the diffusion layer is a first layer of particles and the wavelength is xe2x80x9cxcexxe2x80x9d of the light being incident upon the screen.
D≈xcexxc3x97Fxc3x97Mxe2x80x83xe2x80x83(Eq. 1)
By the way, in a case when the interference range D is fully or sufficiently larger than the particle diameter of the diffusion materials, such as the random diffuser elements, the speckle disturbances can easily occur, on the contrary to this, it hardly occurs when the interference range xe2x80x9cDxe2x80x9d is fully smaller than the particle diameter of the diffusion materials. In the case of the apparatus using the CRTs therein, since the practical values of the wavelength xe2x80x9cxcexxe2x80x9d, the F-number, and the projection magnification xe2x80x9cMxe2x80x9d are about xcex=0.50 xcexcm, F=1.0, and M=11 (in the case of the display size of the CRT being 5 inches, and of the screen size being 55 inches in the opposite angle), respectively, therefore the interference range D comes to be D≈6 xcexcm. This value is smaller than the particle diameter (i.e., several tens xcexcm) of the diffusion materials, which are used actually. Accordingly, in the case of the apparatus of using the CRTs therein, it is difficult that the speckle disturbances occur.
On a while, with the apparatus of using the image modulation element therein, since the practical values of the xe2x80x9cxcexxe2x80x9d, the F-number and the projection magnification xe2x80x9cMxe2x80x9d are about xcex=0.50 xcexcm, F=2.5, and M=70 (in the case of the display size of the image modulation element being about 0.8 inch, and the screen size being 55 inches in the opposite angle), respectively, therefore the interference range xe2x80x9cDxe2x80x9d comes to be D=90 xcexcm. This value is larger than the particle diameter (i.e., several tens xcexcm) of the diffusion materials, which are used actually. Accordingly, in the case of the apparatus of using the image modulation element therein, the speckle disturbances can easily occur because the visual angle xcex2 (=1/Fxc2x7M) of the projection lens is small.
The conventional technology, for the purpose of reducing the speckle disturbances in the case of using the projection lens of a small visual angle xcex2, is described or known by, for example, Japanese Patent Laying-Open No. Hei 11-024169 (1999). In this conventional art, it is disclosed that the Fresnel sheet has the function of a spatial filter, for averaging the images over the area being larger than the interference range xe2x80x9cDxe2x80x9d, in the case where the diffusion layer is formed on the lenticular sheet of the translucent screen.
However, for the purpose of enlarging an angle of field or visibility in the vertical and/or horizontal direction(s), the light diffusion materials are mixed, not only into the lenticular sheet, but also into the Fresnel sheet. In the conventional art mentioned above, no consideration is taken into the consideration, upon the reduction of the speckle disturbances in the case of mixing the light diffusion element or agent into an inside of the Fresnel sheet. Nor no such the technology/idea is proposed, for reducing the speckle disturbances while suppressing the deterioration on picture quality (in particular, on the focus characteristic or property) in such the cases.
Accordingly, the present invention has an object, in the rear-projection display apparatus of using the image modulation element therein, preferably, to reduce the speckle disturbances in the case where the light diffusion materials are mixed into both the lenticular sheet and the Fresnel sheet, while suppressing the deterioration on the picture quality thereof.
For achieving the object mentioned above, in accordance with the present invention, the Fresnel sheet and the lenticular sheet are constructed, so that they satisfy the following conditions, in particular, in the case where the light diffusion materials are mixed into them, respectively:
15xe2x89xa6Hfxc2x7HLxe2x89xa645
xe2x80x83Hfxe2x89xa6HL
Where, Hf (%) is a haze value indicative of content of the diffusion material mixed into the Fresnel sheet mentioned above, while HL (%) the haze value indicative of content of the diffusion material into the lenticular sheet mentioned above.
Namely, in accordance with the present invention, the contents of the light diffusion materials, being mixed in the lenticular sheet and the Fresnel sheet, are set to the values mentioned above, respectively, thereby to satisfy the following condition for reducing the speckle disturbances:
t0xc2x7xcex8v greater than D
Where, xe2x80x9ct0xe2x80x9d indicates the distance from the light incident surface of the Fresnel sheet mentioned above, up to the focus of the lenticular lens formed on the lenticular sheet mentioned above, xe2x80x9cxcex8vxe2x80x9d a composed value between light emission angles, including xe2x80x9cxcex8vfxe2x80x9d of the Fresnel sheet mentioned above and xe2x80x9cxcex8vHxe2x80x9d of the lenticular sheet mentioned above, and xe2x80x9cDxe2x80x9d the interference range defined between the light diffusion member and the light, being obtained by the product among the F-number xe2x80x9cFxe2x80x9d, the magnification xe2x80x9cMxe2x80x9d of the projection lens mentioned above, and the wavelength xe2x80x9cxcexxe2x80x9d of the light, which is incident upon the translucent screen mentioned above.
With such the structure mentioned above, in accordance with the present invention, it is possible to reduce the speckle disturbances of causing the deterioration on the picture quality, while maintaining high rightness and high focuses the display apparatus.